Cooling devices of this type can be used in rotary piston compressors for refrigeration and air-conditioning systems, for example in a screw-type compressor with oil injection. Refrigeration and air-conditioning systems essentially comprise an evaporator in which heat is withdrawn from the environment by evaporation of the coolant, a compressor which increases the pressure of the vaporized coolant from a suction pressure to an outlet pressure, and a condenser in which the vaporized coolant under the discharge pressure is liquefied again under heat emission.
In screw-type compressors, two screw-like rotors meshing with each other are arranged within the compressor casing for compressing the coolant, these rotors being tightly sealed radially by the compressor casing. In most cases, the screw-type compressors or worm compressors used in refrigeration systems have a means for oil injection. The oil is injected into the compression spaces of the screw-type compressors and thus into the gas to be compressed which is located therein. This serves essentially the following three purposes:
1. For cooling the compression process: By means of the injected oil, the coolant to be compressed is cooled and with that, the screw-type compressor as a whole is also cooled. It is thereby subjected to smaller temperature differences. This means that fittings and tolerances can be carried out more closely, whereby the clearance losses in the compressor are decreased. PA1 2. For lubricating the rotors and the bearings: Since in known oil-injected screw-type compressors only one of the rotors is usually driven externally, for example by an electric motor or the like, the other rotor must be driven indirectly by and together with the driven rotor. The injected oil thereby decreases the wear and tear at the two rotors. Besides this, the oil is used for lubricating the bearings of the rotors. PA1 3. For sealing the clearances within the compression space: The injected oil seals the clearances between the two rotors and between the individual rotors and the compressor casing. In this manner, possibly existing leakage paths within the compressor are sealed and thus the conditions for a high degree of efficiency of the compressor are created. The oil injected into the compression chamber is vaporized and is carried along by the gaseous coolant to be compressed which is located in the compression chamber. Thus, there is an oil-coolant mixture at the pressure outlet of the compressor. The oil found in the oil-coolant mixture must be separated from the coolant by oil separators in order to be injected again into the compressor and so that heat transmissions of the coolant within the refrigerating circuit are not influenced unfavourably.
When compressing to high pressures, the oil injected into the compressor is cooled as a function of a final temperature resulting at the pressure outlet of the compressor. Thereby, cooling can take place by means of coolant injection, or by cooling with water or with air in a heat exchanger, e.g. a plate-type heat exchanger. A large quantity of injected oil necessitates large and expensive heat exchangers in the latter case.
The temperature of the injected oil is essentially determined in that its viscosity is great enough to ensure lubrication of the bearings. When the oil temperature increases, then the viscosity of the oil decreases and the lubrication of the bearings of the rotors is endangered. For the above-mentioned sealing of the clearances, which requires the greatest quantity of injected oil, also lower oil viscosities or higher oil temperatures, however, would be allowable.